1. Field of the Invention
The invention relates to a control device for a variable displacement hydrostatic machine having a stroke-volume regulator to which an actuating pressure can be applied. The actuating pressure is the delivery pressure of the machine and, if the delivery pressure of the machine is insufficient, a pressure produced by a booster pump is the actuating pressure.
2. Description of Related Prior Art
U.S. Pat. No. 3,846,982 describes a hydrostatic drive for a vehicle in the form of a hydraulic pump having an adjustable stroke-volume. The adjustment of the stroke-volume is accomplished by an actuating pressure which, in the neutral position of the hydraulic pump, is produced by an auxiliary pump, which at the same time is the feed pump for a closed hydrostatic cycle so that the auxiliary pump always feeds into the delivery-pressure line carrying the lower pressure. When the hydraulic pump is not in the neutral position, the pressure in the high-pressure cycle of the machine is used as the actuating pressure.
A disadvantage of using the high-pressure cycle of the machine as the actuating pressure is that the high pressure creates large forces which have to be absorbed at limit stops in the stroke-volume control. In addition, the regulating forces and the regulating times vary greatly because of the large pressure differential between the minimum actuating pressure, i.e., the feed pressure, and the maximum actuating pressure, i.e., the highest pressure in the delivery line of the pump. If the high pressure is used for other control purposes, for example, to cut off pressure or to limit output, numerous lines are generally required to pass the high pressure to the control system.
To avoid these problems, it is known to provide low-pressure regulation instead of high-pressure regulation. However, low-pressure regulation has the disadvantage that large pistons are required to produce the regulating force, so that the machine must be relatively large.